Photonic integrated circuits (PICs) based on near-infrared (NIR) light include active and passive portions of a single chip and are used to produce off-the-shelf integrated devices. NIR PICs typically incorporate diode lasers, which are bipolar devices that utilize interband transitions between the valence and conduction bands. Diode lasers consist of both n- and p-type layers, and the emission wavelength is directly related to the bandgap of the active regions in the semiconductor structure. PICs that utilize diode lasers may be fabricated by increasing a bandgap of one or more semiconductor layers to minimize inter-band transitions in a passive waveguide region. Alternatively, NIR PICs may be fabricated by etching away the original waveguide from a region intended to be passive, followed by regrowing new layers having a larger bandgap than the laser wavelength and a low free-carrier concentration.
In contrast to diode lasers, quantum cascade lasers (QCLs) and other quantum cascade (QC) devices are unipolar devices that utilize intra-band transitions between subbands within the conduction band. The emission photon energy is significantly lower than the energy bandgap of the semiconductor materials that comprise the QC structure. QC devices do not rely on inter-band transitions, and they typically do not utilize two different types of doping (n- and p-type).